1. Field of the Invention
The invention relates to a hydropneumatic suspension for vehicles having greatly varying axle loads.
2. Description of Related Art
To improve the driving comfort of vehicles having greatly varying axle loads, it is desirable to adjust the front axle suspension performance to the different axle loads. Depending on the application, the front axle of a vehicle of this type, can be exposed to very low and very high static loads. Level regulators, which adjust the height of the suspended masses to that of the unsuspended masses by opening and closing directional control valves, are used in hydropneumatic suspension systems. In the dual-action hydraulic cylinders used in the suspension system, the cylinder pistons are kept at a medium level by regulating the level up or down.
A hydropneumatic suspension for motor vehicles having a wide range of axle loads is known from German Patent Application 41 20 758 A1, which uses dual-action hydraulic cylinders whose cylinder spaces are connected to a reservoir and whose annular spaces on the piston rod side are connected to an additional accumulator. A level-regulating valve regulates the height position, while a pressure-controlled valve regulates a preset pressure ratio between pressures in the first and second reservoirs as a function of the load on the hydropneumatic actuators. The pressure-controlled valve is actuated by both the pressure in the pressure line leading to the cylinder spaces and the pressure in the pressure line leading to the annular spaces.
German Patent 42 42 448 C1 describes another pressure regulation method which advantageously uses load-sensing pumps. This arrangement uses a pressure regulating valve that maintains a constant pressure level in the annular space of the suspension cylinder. The corresponding pressure level is assigned to the tractor load.
A device according to German Patent Application 197 19 077 A1 provides an especially advantageous way to improve driving comfort by using a load-adjusted two-way pressure regulating valve. This makes it possible to regulate even high load ratios and adjust the suspension to the load situations. The annular space pressure can be adjusted between the upper and lower limit value depending on the load.
The device according to German Patent Application 197 19 076 A1 differs from the one described in German Patent Application 197 19 077 A1 in that it uses an electric height regulator whose electronically processed control signals control solenoid valves.
Experience has shown that a two-way pressure regulating valve can adequately adjust the comfort level in tractor suspension systems. However, if a more sensitive adjustment is desired, taking additional dependencies into account, a multi-way pressure regulating valve is required, which is very expensive to implement according to the known functional principle.
It is an object of the invention to provide a control system that makes it possible to optimally adjust tractor driving comfort and performance to the conditions of the application at hand. There should not be any abrupt changes in suspension performance. The driver should feel an adequately gentle suspension action with uniform amplitudes in all conceivable driving situations.
In a hydropneumatic suspension for vehicles having greatly varying axle loads, in particular tractors having hydraulic suspension cylinders that are connectable to a pneumatic feed pump via supply linesxe2x80x94with a pressure regulating valve being provided in the supply line leading to the annular spaces of the suspension cylindersxe2x80x94these and other objects of the invention are achieved by designing the pressure regulating valve as a proportionally regulated valve which is controlled by a control current from an electric control unit and electronically processing the measured signals of a pressure sensor connected to the piston chambers of the suspension cylinders or linking them to additional functional data or to external control signals from the working hydraulics.
Achieving the object in this manner not only provides good adaptability of the spring rates, but also advantageously eliminates the need for control elements and connecting lines that are generally provided to operate the pressure regulating valve. The main advantage, however, is that the axle spring rate is continuously adjusted to the suspended axle load without interruption, thus making it possible to select any curve shape for the axle spring rate as a function of axle load and other operating states. As the servomotor adjusts the pressure regulating valve, the valve regulating spring is continuously adjusted to the pressure prevailing in the supply line leading to the piston chambers of the suspension cylinders, thereby continuously regulating the pressure in the annular spaces of the suspension cylinders.